This one is super simple and I use it all the time. It's not that I have weak arms, it's that I have slippery hands. The traction created by the rubber bands will make the lid twist off easily, even if your hands are wet or recently moisturized.
A jar that you can't open
1-2 rubber bands (the thick ones work the best)
1. Wrap the rubber band around the lid of the jar. Twist the lid off.
2. If your jar is particularly difficult, wrap the second rubber band around the jar itself. Twist the lid off.